The heart of a computer's long term memory is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, or spacer layer, sandwiched between first and second ferromagnetic layers, referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is oriented generally perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is oriented generally parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos θ, where θ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
However, a significant problem presented by such GMR sensors is that of excessive magnetic noise or signal noise, which limits ability of GMR sensors to achieve ever increasing data rates and areal densities needed for current and future data recording applications. Magnetic noise arises from the thermal fluctuations of the free layer at the sensor ambient temperature, as the magnetic order competes with the thermally-induced magnetic disorder. As the sensor gets smaller, this magnetic noise increases. It is already a significant part of the signal noise budget in GMR sensors and will play an ever increasing role as devices are made smaller.
An important class of potential magnetoresistive sensors, magnetic recording sensors and scanning sensors, called Lorentz Magnetoresistors, rely on the Lorentz force resulting from the motion of a charged carrier in a magnetic field. One type of such devices is called a Hall sensor. Another is what has been called an Extraordinary Magnetoresistive Sensor (EMR). An advantage of these sensors is that the active region of the sensor is constructed of non-magnetic semiconductor materials, and does not suffer from the problem of magnetic noise that exists in giant magnetoresistive sensors (GMR) and tunnel valves, both of which use magnetic films in their active regions.
The EMR sensor includes a pair of voltage leads and a pair of current leads in contact with one side of the active region and an electrically conductive shunt in contact with the other side of the active region. In the absence of an applied magnetic field, sense current through the current leads passes into the semiconductor active region and is shunted through the shunt structure. When an applied magnetic field is present, current is deflected from the shunt and passes primarily through the semiconductor active region, thereby increasing the electrical resistance of the device. This change in electrical resistance in response the applied magnetic field is detected across the voltage leads. EMR is described by T. Zhou et al., “Extraordinary magnetoresistance in externally shunted van der Pauw plates”, Appl. Phys. Lett., Vol. 78, No. 5, 29 Jan. 2001., pp. 667-669.
As disclosed in U.S. Patent Application US20060018054A1, entitled NARROW TRACK EMR DEVICE, such EMR devices provide a pathway for producing narrow track sensors with unprecedented spatial resolution requiring no shields. However, the signal amplitude in response to nanoscale magnetic domains is expected to be small, rendering signal detection and processing with high Signal to Noise Ratio (SNR) challenging, thereby precluding the use of EMR sensors for recording densities beyond 1 Tb/in2.
Therefore there is a strong felt need for a mechanism for efficiently amplifying a signal from an Lorentz magnetoresistive device without introducing undue amounts of noise into the signal to be detected. Such a design, method or structure would allow such magnetoresistive structures to be used in future high data density recording devices.